Method for transmitting and receiving downlink control information in a wireless communication system and apparatus for the same

ABSTRACT

A method for receiving downlink channels from a base station (BS) at a user equipment (UE) in a wireless communication system. Control information is detected by monitoring a physical downlink control channel (PDCCH) or an enhanced physical downlink control channel (EPDCCH). A broadcast channel or a downlink shared channel is received based on the control information. The PDCCH is monitored on subframes configured for the broadcast channel. The EPDCCH is not monitored on the subframes configured for the broadcast channel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 14/003,425 filed on Sep. 5, 2013, which is the national phaseof PCT International Application No. PCT/KR2012/001248 filed on Feb. 20,2012, which claims the benefit of U.S. Provisional Application No.61/480,374 filed on Apr. 29, 2011. The entire contents of all of theabove applications are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wireless communication system, andmore particularly, to a method for transmitting and receiving downlinkcontrol information in a wireless communication system and an apparatusfor the same.

2. Discussion of the Related Art

A 3^(rd) generation partnership project long term evolution(hereinafter, referred to as ‘LTE’) communication system which is anexample of a wireless communication system to which the presentinvention can be applied will be described in brief.

FIG. 1 is a diagram illustrating a network structure of an EvolvedUniversal Mobile Telecommunications System (E-UMTS) which is an exampleof a wireless communication system. The E-UMTS system is an evolvedversion of the conventional UMTS system, and its basic standardizationis in progress under the 3rd Generation Partnership Project (3GPP). TheE-UMTS may also be referred to as a Long Term Evolution (LTE) system.For details of the technical specifications of the UMTS and E-UMTS,refer to Release 7 and Release 8 of “3rd Generation Partnership Project;Technical Specification Group Radio Access Network”.

Referring to FIG. 1, the E-UMTS includes a User Equipment (UE), basestations (eNode B and eNB), and an Access Gateway (AG) which is locatedat an end of a network (E-UTRAN) and connected to an external network.The base stations may simultaneously transmit multiple data streams fora broadcast service, a multicast service and/or a unicast service.

One or more cells may exist for one base station. One cell is set to oneof bandwidths of 1.25, 2.5, 5, 10, and 20 MHz to provide a downlink oruplink transport service to several user equipments. Different cells maybe set to provide different bandwidths. Also, one base station controlsdata transmission and reception for a plurality of user equipments. Thebase station transmits downlink (DL) scheduling information of downlinkdata to the corresponding user equipment to indicate time and frequencydomains to which data will be transmitted and information related toencoding, data size, hybrid automatic repeat and request (HARQ). Also,the base station transmits uplink (UL) scheduling information of uplinkdata to the corresponding user equipment to indicate time and frequencydomains that can be used by the corresponding user equipment, andinformation related to encoding, data size, HARQ. An interface fortransmitting user traffic or control traffic may be used between thebase stations. A Core Network (CN) may include the AG and a network nodeor the like for user registration of the user equipment. The AG managesmobility of the user equipment on a Tracking Area (TA) basis, whereinone TA includes a plurality of cells.

Although the wireless communication technology developed based on WCDMAhas been evolved into LTE, request and expectation of users andproviders have continued to increase. Also, since another wirelessaccess technology is being continuously developed, new evolution of thewireless communication technology is required for competitiveness in thefuture. In this respect, reduction of cost per bit, increase ofavailable service, use of adaptable frequency band, simple structure,open type interface, proper power consumption of user equipment, etc.are required.

SUMMARY OF THE INVENTION

Accordingly, based on the aforementioned discussion, an object of thepresent invention is to provide a method for transmitting and receivingdownlink control information in a wireless communication system and anapparatus for the same, which substantially obviate one or more problemsdue to limitations and disadvantages of the related art.

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein, inone aspect of the present invention, a method for receiving a downlinksignal in a user equipment of a wireless communication system from abase station comprises the steps of receiving control information fromeither a downlink control channel or a legacy downlink control channelin accordance with a previously set condition; and receiving a downlinkshared channel or a legacy downlink shared channel on the basis of thecontrol information, wherein the downlink control channel is receivedthrough a data region of a subframe, and the legacy downlink controlchannel is received through a control region of the subframe, thecontrol information of the downlink control channel includes allocationinformation of the downlink shared channel, and the control informationof the legacy downlink control channel includes allocation informationof the downlink shared channel or the legacy downlink shared channel.

The previously set condition means a case where signal quality of thedownlink control channel is less than a threshold value, a case wheredecoding of the downlink control channel is failed within the range thatexceeds the number of previously set times, or a case where decoding ofthe downlink control channel is failed for more than a subframe which ispreviously set. if the previously set condition is satisfied, thecontrol information is received from the legacy downlink controlchannel.

In another aspect of the present invention, a user equipment in awireless communication system comprises a wireless communication modulefor receiving a downlink control channel or a legacy downlink controlchannel from a base station in accordance with a previously setcondition; and a processor for acquiring control information by decodingthe received downlink control channel or legacy downlink controlchannel, wherein the wireless communication module receives a downlinkshared channel or a legacy downlink shared channel from the base stationon the basis of the control information, receives the downlink controlchannel through a data region of a subframe and receives the legacydownlink control channel through a control region of the subframe, andthe control information of the downlink control channel includesallocation information of the downlink shared channel, and the controlinformation of the legacy downlink control channel includes allocationinformation of the downlink shared channel or the legacy downlink sharedchannel.

The processor controls the wireless communication module to receive thelegacy downlink control channel if signal quality of the downlinkcontrol channel is less than a threshold value, if decoding of thedownlink control channel is failed within the range that exceeds thenumber of previously set times, or if decoding of the downlink controlchannel is failed for more than a subframe which is previously set.

The control information included in the legacy downlink control channelmay be the same as the control information included in the downlinkcontrol channel of which decoding has been failed.

Also, the legacy downlink control channel is transmitted from asubframe, which is previously set, in a unit of a specific number ofradio frames. Preferably, the subframe, is previously set, is thesubframe to which broadcast information is transmitted.

According to the embodiment of the present invention, the user equipmentcan effectively receive a downlink signal in a wireless communicationsystem.

It is to be understood that the advantages that can be obtained by thepresent invention not limited to the aforementioned advantages and otheradvantages which are not mentioned will be apparent from the followingdescription to the person with an ordinary skill in the art to which thepresent invention pertains.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a network structure of an EvolvedUniversal Mobile Telecommunications System (E-UMTS) which is an exampleof a mobile communication system;

FIG. 2 is a diagram illustrating structures of a control plane and auser plane of a radio interface protocol between a user equipment andE-UTRAN based on the 3GPP radio access network standard;

FIG. 3 is a diagram illustrating physical channels used in a 3GPP systemand a general method for transmitting a signal using the physicalchannels;

FIG. 4 is a diagram illustrating a structure of a radio frame used in a3GPP system;

FIG. 5 is a diagram illustrating a structure of a downlink radio frameused in an LTE system;

FIG. 6 is a diagram illustrating features of an E-PDCCH;

FIG. 7 is a diagram illustrating an example that a normal mode and afallback mode are set in accordance with the embodiment of the presentinvention;

FIG. 8 is a diagram illustrating an example for dividing a case where aPDCCH is used from a case where an E-PDCCH is used depending on featuresof control information in accordance with the embodiment of the presentinvention; and

FIG. 9 is a block diagram illustrating a communication apparatusaccording to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, structures, operations, and other features of the presentinvention will be understood readily by the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. Embodiments described later are examples in which technicalfeatures of the present invention are applied to a 3GPP system.

In this specification, although the embodiment of the present inventionwill be described based on the LTE system and the LTE-A system, the LTEsystem and the LTE-A system are only exemplary and the present inventionmay be applied to all communication systems corresponding to theaforementioned definition.

FIG. 2 is a diagram illustrating structures of a control plane and auser plane of a radio interface protocol between a user equipment andE-UTRAN based on the 3GPP radio access network standard. The controlplane means a passageway where control messages are transmitted, whereinthe control messages are used by the user equipment and the network tomanage call. The user plane means a passageway where data generated inan application layer, for example, voice data or Internet packet dataare transmitted.

A physical layer as the first layer provides an information transferservice to an upper layer using a physical channel. The physical layeris connected to a medium access control layer above the physical layervia a transport channel. Data are transferred between the medium accesscontrol layer and the physical layer via the transport channel. Data aretransferred between one physical layer of a transmitting side and theother physical layer of a receiving side via the physical channel. Thephysical channel uses time and frequency as radio resources. In moredetail, the physical channel is modulated in accordance with anorthogonal frequency division multiple access (OFDMA) scheme in adownlink, and is modulated in accordance with a single carrier frequencydivision multiple access (SC-FDMA) scheme in an uplink.

A medium access control layer of the second layer provides a service toa radio link control (RLC) layer above the MAC layer via logicalchannels. The RLC layer of the second layer supports reliable datatransfer. The RLC layer may be implemented as a functional block insidethe MAC layer. In order to effectively transmit IP packets such as IPv4or IPv6 within a radio interface having a narrow bandwidth, a packetdata convergence protocol (PDCP) layer of the second layer performsheader compression to reduce the size of unnecessary controlinformation.

A radio resource control (RRC) layer located on the lowest part of thethird layer is defined in the control plane only. The RRC layer isassociated with configuration, re-configuration and release of radiobearers to be in charge of controlling the logical, transport andphysical channels. In this case, the radio bearer (RB) means a serviceprovided by the second layer for the data transfer between the userequipment and the network. To this end, the RRC layers of the userequipment and the network exchange RRC message with each other. If theRRC layer of the user equipment is RRC connected with the RRC layer ofthe network, the user equipment is in RRC connected mode. If not so, theuser equipment is in RRC idle mode. A non-access stratum (NAS) layerlocated above the RRC layer performs functions such as sessionmanagement and mobility management.

One cell constituting a base station (eNB) is set to one of bandwidthsof 1.25, 2.5, 5, 10, 15, and 20 MHz and provides a downlink or uplinktransmission service to several user equipments. At this time, differentcells may be set to provide different bandwidths.

As downlink transport channels carrying data from the network to theuser equipment, there are provided a broadcast channel (BCH) carryingsystem information, a paging channel (PCH) carrying paging message, anda downlink shared channel (SCH) carrying user traffic or controlmessages. Traffic or control messages of a downlink multicast orbroadcast service may be transmitted via the downlink SCH or anadditional downlink multicast channel (MCH). Meanwhile, as uplinktransport channels carrying data from the user equipment to the network,there are provided a random access channel (RACH) carrying an initialcontrol message and an uplink shared channel (UL-SCH) carrying usertraffic or control message. As logical channels located above thetransport channels and mapped with the transport channels, there areprovided a broadcast control channel (BCCH), a paging control channel(PCCH), a common control channel (CCCH), a multicast control channel(MCCH), and a multicast traffic channel (MTCH).

FIG. 3 is a diagram illustrating physical channels used in a 3GPP systemand a general method for transmitting a signal using the physicalchannels.

The user equipment performs initial cell search such as synchronizingwith the base station when it newly enters a cell or the power is turnedon (S301). To this end, the user equipment synchronizes with the basestation by receiving a primary synchronization channel (P-SCH) and asecondary synchronization channel (S-SCH) from the base station, andacquires information of cell ID, etc. Afterwards, the user equipment mayacquire broadcast information within the cell by receiving a physicalbroadcast channel from the base station. Meanwhile, the user equipmentmay identify the status of a downlink channel by receiving a downlinkreference signal (DL RS) at the initial cell search step.

The user equipment which has finished the initial cell search mayacquire more detailed system information by receiving a physicaldownlink shared channel (PDSCH) in accordance with a physical downlinkcontrol channel (PDCCH) and information carried in the PDCCH (S302).

Meanwhile, if the user equipment initially accesses the base station, orif there is no radio resource for signal transmission, the userequipment may perform a random access procedure (RACH) for the basestation (S303 to S306). To this end, the user equipment may transmit apreamble of a specific sequence through a physical random access channel(PRACH) (S303 and S305), and may receive a response message to thepreamble through the PDCCH and the PDSCH corresponding to the PDCCH(S304 and S306). In case of a contention based RACH, a contentionresolution procedure may be performed additionally.

The user equipment which has performed the aforementioned steps receivesthe PDCCH/PDSCH (S307) and transmits a physical uplink shared channel(PUSCH) and a physical uplink control channel (PUCCH) (S308), as ageneral procedure of transmitting uplink/downlink signals. Inparticular, the user equipment receives downlink control information(DCI) through the PDCCH. In this case, the DCI includes controlinformation such as resource allocation information for the userequipment, and has different formats depending on its usage.

In the mean time, control information transmitted from the userequipment to the base station or received from the base station to theuser equipment through the uplink includes downlink/uplink ACK/NACKsignals, a channel quality indicator (CQI), a preceding matrix index(PM), and a rank indicator (RI). In case of the 3GPP LE system, the userequipment may transmit the aforementioned control information such asCQI/PMI/RI through the PUSCH and/or the PUCCH.

FIG. 4 is a diagram illustrating a structure of a radio frame used in anLTE system.

Referring to FIG. 4, the radio frame has a length of 10 ms(327200·T_(s)) and includes 10 subframes of an equal size. Each subframe has a length of 1 ms and includes two slots. Each slot has alength of 0.5 ms (15360·T_(s)). In this case, T_(s) represents asampling time, and is expressed by T_(s)=1/(15 kHz×2048)=3.2552×10⁻⁸(about 33 ns). The slot includes a plurality of OFDM symbols in a timedomain, and includes a plurality of resource blocks (RBs) in a frequencydomain. In the LTE system, one resource block includes twelve (12)subcarriers×seven (or six) OFDM symbols. A transmission time interval(TTI), which is a transmission unit time of data, may be determined in aunit of one or more subframes. The aforementioned structure the radioframe is only exemplary, and various modifications may be made in thenumber of subframes included in the radio frame or the number of slotsincluded in the subframe, or the number of OFDM symbols included in theslot.

FIG. 5 is a diagram illustrating a control channel included in a controlregion of one subframe in a downlink radio frame.

Referring to FIG. 5, the subframe includes fourteen (14) OFDM symbols.First one to three OFDM symbols are used as the control region inaccordance with subframe configuration, and the other thirteen to elevenOFDM symbols are used as the data region. In FIG. 5, R1 to R4 representreference signals (RS) (or pilot signals) of antennas 0 to 3. The RS isfixed by a given pattern within the subframe regardless of the controlregion and the data region. The control channel is allocated to aresource to which the RS is not allocated in the control region, and atraffic channel is also allocated to a resource to which the RS is notallocated in the data region. Examples of the control channel allocatedto the control region include a Physical Control Format IndicatorChannel (PCFICH), a Physical Hybrid-ARQ Indicator Channel (PHICH), and aPhysical Downlink Control Channel (PDCCH).

The PCFICH notifies the user equipment of the number of OFDM symbolsused in the PDCCH per subframe. The PCFICH is located in the first OFDMsymbol and configured. prior to the PHICH and the PDCCH. The PCFICHincludes four resource element groups (REG), each REG being distributedin the control region based on cell identity (cell ID). One REG includesfour resource elements (REs). The RE represents a minimum physicalresource defined by one subcarrier×one OFDM symbol. The PCFICH valueindicates a value of 1 to 3 or a value of 2 to 4 depending on abandwidth, and is modulated by Quadrature Phase Shift Keying (QPSK).

The PHICH is a physical hybrid-automatic repeat and request (HARQ)indicator channel and is used to carry HARQ ACK/NACK signals for uplinktransmission. Namely, the PHICH represents a channel where DL ACK/NACKinformation for UL HARQ is transmitted. The PHICH includes one REG, andis cell-specifically scrambled. The ACK/NACK signals are indicated by 1bit, and are modulated by binary phase shift keying (BPSK). Themodulated ACK/NACK are spread by a spreading factor (SF)=2 or 4. Aplurality of PHICHs may be mapped with the same resource and constitutea PHICH group. The number of PHICHs multiplexed in the PHICH group isdetermined by the number of spreading codes. The PHICH (group) isrepeated three times to obtain diversity gain in the frequency domainand/or the time domain.

The PDCCH is allocated to first n number of OFDM symbols of thesubframe, wherein n is an integer greater than 1 and is indicated by thePCIFCH. The PDCCH includes one or more CCEs. The PDCCH notifies eachuser equipment or user equipment group of information related toresource allocation of transport channels, i.e., a paging channel (PCH)and a downlink-shared channel (DL-SCH), uplink scheduling grant, HARQinformation, etc. The paging channel (PCH) and the downlink-sharedchannel (DL-SCH) are transmitted through the PDSCH. Accordingly, thebase station and the user equipment respectively transmit and receivedata through the PDSCH except for specific control information orspecific service data.

Information as to user equipment(s) (one user equipment or a pluralityof user equipments) to which data of the PDSCH are transmitted, andinformation as to how the user equipment(s) receives and decodes PDSCHdata are transmitted by being included in the PDCCH. For example, it isassumed that a specific PDCCH is CRC masked with radio network temporaryidentity (RNTI) called “A,” and information of data transmitted using aradio resource (for example, frequency location) called “B” andtransmission format information (for example, transport block size,modulation mode, coding information, etc.) called “C” is transmittedthrough a specific subframe. In this case, one or more user equipmentslocated in a corresponding cell monitor the PDCCH using their RNTIinformation, and if there are one or more user equipments having RNTIcalled “A”, the user equipments receive the PDCCH, and receive the PDSCHindicated by “B” and “C” through information of the received PDCCH.

Meanwhile, in the 3GPP LTE system, a downlink subframe mainly includes aPDCCH and a PDSCH, and if a cell B corresponds to the same LTE system, acell A and subframe format are used. In order that downlink transmissionfrom the cell A avoids interference downlink transmission from the cellB, it may be considered that the cell A and the cell B respectivelyperform transmission from their respective subframes different from eachother. Namely, limited subframe resources are used divisionally by atime division mode. An almost blank subframe (ABS) which is beingrecently discussed is an example of the aforementioned interferenceavoidance, and a subframe designated as the ABS is set to transmit acommon reference signal (CRS) only. However, this greatly reducesresource usage efficiency. Accordingly, it may be more considered thatthe two cells share the same subframe if possible but frequencyresources are divided from space resources. Alternatively, it may beconsidered that these two approach methods may efficiently be combinedwith each other.

As another method for reducing inter-cell interference, it may beconsidered a method for minimizing interference on the cell B byspatially dividing a data channel of the cell A and arranging thedivided channels at different directions from each other. In otherwords, precoding is applied to the cell A to avoid interference and atthe same time direct a transport beam in a corresponding direction,whereby the transport beam is normally transmitted to a specific userequipment. As a result, interference of the data channel of the cell Aon a data channel of the cell B may be reduced remarkably.

However, a control channel is generally transmitted over an entiresystem band and decoded using a common reference signal (CRS).Accordingly, if precoding is applied to the control channel, informationon precoding performed for detection and decoding by a receiver isrequired. Since the precoding information is transmitted through thecontrol channel, the control channel cannot be decoded. Accordingly, todecode the control channel, a separate reference signal for normallydecoding the control channel is required. In this case, complexity intransmission and reception may be increased.

In this respect, as another method for avoiding inter-cell interference,the control channel such as the PCFICH or the PHICH is decoded using theCRS in the same manner as the existing PDCCH. However, different methodsshould be used depending on types of the control channels. For example,the PDCCH may be transmitted to a specific position of a PDSCH resourceregion, and the PCFICH and the PHICH may be transmitted to either thefirst symbol or the first to fourth symbols.

Also, it may be considered that the PCFICH, the PHICH and the CRS arelocated at the first OFDM symbol and the PDCCH is transmitted after thethird symbol. Uplink grant or downlink grant transmitted through thePDCCH may be located at the second slot not the first slot.Alternatively, the downlink grant may be located at the first slot onlyand the uplink grant may be located at the second slot only. It isassumed that important system information, i.e., BCH, PCH and SCHtransmitted through a broadcasting format is managed in the same manneras the existing method.

In this case, a control channel newly suggested unlike the existingPDCCH is referred to as an enhanced PDCCH or E-PDCCH. The E-PDCCH ischaracterized in that a CRS based transmit diversity or spatialmultiplexing (SM) scheme may be applied to the E-PDCCH and that theE-PDCCH may be operated based on a DM-RS which is a user equipmentspecific reference signal. The same precoding may be applied to theE-PDCCH and the PDSCH, whereby interference may be minimized at thecorresponding subframe of the cell B.

The control information such as the POACH located at the first symbol ofthe cell A may still cause interference. However, beamforming may beperformed for the PDCCH and the PDSCH, which are main factors ofinterference, whereby interference may be reduced remarkably. Also,since the PCFICH performs inter-cell hopping based on cell ID,robustness a little exists in inter-cell interference.

Hereinafter, detailed features of the E-PDCCH will be described based onthe aforementioned concept.

FIG. 6 is a diagram illustrating features of an E-PDCCH. Particularly,in FIG. 6, a region for the existing PDCCH and a PDSCH corresponding tothe existing PDCCH are shown as compared with a region for the E-PDCCHand an E-PDSCH region corresponding to the E-PDCCH.

Referring to FIG. 6, lengths of frequency and time domains into whichthe E-PDCCH is mapped may be set in various manners. For example, theE-PDCCH may be configured from the fourth symbol to the last symbol, maybe configured only after the fourth symbols of the first slot, or may beconfigured at the second slot only. Also, positions of DL grant and ULgrant for the E-PDCCH may be configured in various manners. However, itis important that the E-PDCCH is designed so as not to overlap theexisting PDCCH region, whereby interference does not occur.

The aforementioned E-PDCCH and E-PDSCH have a structural feature thatcan transmit data region control information unlike the existingsubframe structure where control information is transmitted to a controlregion. This structural feature may be used to reduce inter-cellinterference in a heterogeneous wireless network system that includes amacro cell and a pico cell as described above. For example, if an MBSFNsubframe where control information and CRS exist at first two symbolsonly is set to ABS, the other region except for the first two symbolsmay be set to a subframe having no interference.

In a resource region where interference is limited, control informationand data are preferably configured to be safely transmitted. Forexample, a data region where the E-PDCCH may exist, i.e., search spacemay previously be designated through RRC signaling, etc., and blinddecoding may be performed for the data region only to decode theE-PDCCH.

However, unexpected interference may still occur in the data region, andthe E-PDCCH may not be decoded normally due to change of search spaceconfiguration for the E-PDCCH. In this case, the system is preferablydesigned for robust operation that the PDCCH not the E-PDCCH is directlydecoded to acquire resource information for the PDSCH.

Accordingly, the present invention suggests that a mode (hereinafter,referred to as fallback mode) for receiving a PDSCH or E-PDSCH bydecoding a PDCCH, in addition to a mode (hereinafter, referred to asnormal mode) for receiving E-PDSCH by decoding E-PDCCH.

In more detail, E-PDCCH based data transmission is performed in thenormal mode, and the normal mode is switched to the PDCCH based fallbackmode in a specific status to perform data transmission. Also, to switchthe normal mode to the fallback mode, a subframe that can performblinding decoding for the PDCCH should be designated. If the E-PDCCH isnot received or decoding is failed, necessary information is obtainedfrom the subframe that can perform blind decoding for the PDCCH.Although data transmission through the PDCCH may be performed for thesome message as that of the E-PDCCH, it may be configured that data maybe transmitted together with changed resource allocation information,etc.

As examples of the specific status, it may be considered a case whereE-PDCCH receiving quality becomes lower than a threshold value, a casewhere E-PDCCH decoding failure or receiving failure is continued for adesignated time period N times or more, and a case where N subframe orpreviously set time passes after E-PDCCH decoding failure occurs.

Also, a subframe that can perform blind decoding for the PDCCH in thefallback mode may be designated in a radio frame unit, or a specificsubframe may be designated per integer multiple of a radio frame.Alternatively, a subframe (for example, SF#0 or SF#5) to which abroadcast channel is transmitted, or its associated subframe ay beconfigured, and a specific subframe or subframe pattern may previouslybe designated by RRC signaling.

FIG. 7 is a diagram illustrating an example that a normal mode and afallback mode are set in accordance with the embodiment of the presentinvention.

Referring to FIG. 7, a subframe operated in a normal mode, whichreceives E-PDSCH by decoding E-PDCCH, and a fallback subframe operatedin a PDCCCH based fallback mode are set. In particular, the fallbacksubframe is the subframe that is difficult to receive the E-PDCCH orscheduled so as not to receive the E-PDCCH. This fallback subframedemodulates the PDSCH or the E-PDSCH by decoding the PDCCH.

In the mean time, the case where PDCCH is used and the case the E-PDCCHis used may be considered depending on features of the transmittedcontrol information.

FIG. 8 is a diagram illustrating an example for dividing a case where aPDCCH is used from a case where an E-PDCCH is used depending on featuresof control information in accordance with the embodiment of the presentinvention.

Referring to FIG. 8, at a specific subframe (for example, SF#0 or SF#5),system information, information related to change and update ofimportant information such as cell selection/reselection, or otherbroadcast information (e.g., MIB message, SIB1 message, and SI messages)is transmitted through the PDCCH, and dynamic scheduling informationsuch as uplink grant or downlink resource allocation information andinformation related to the dynamic scheduling information aretransmitted through the E-PDCCH other than the specific subframe.

In this case, it is preferable that MIB message, SIB1 message, and SImessages are transmitted from a common search space of the LTE-A systemthrough the PDCCH masked with SI-RNTI, P-RNTI, RA-RNTI, etc. Theinformation transmitted through the PDCCH is not transmitted through theE-PDCCH. Also, as shown in FIG. 8, it is preferable that the searchspace for the E-PDCCH is configured by a dedicated search space not acommon search space.

However, the search space for the E-PDCCH does not exclude that both thecommon search space and the dedicated search space exist. In this case,the designated specific subframe (SF#0 or SF#5) that receives theaforementioned important information may be set to acquire thecorresponding important information by performing blind decoding in thecommon search space of the PDCCH not the common search space of theE-PDCCH. Likewise, dynamic scheduling information is transmitted fromthe dedicated search space for the E-PDCCH. In this case, there is nochange in complexity of blind decoding.

FIG. 9 is block diagram illustrating a communication apparatus accordingto one embodiment of the present invention.

Referring to FIG. 9, the communication apparatus 900 includes aprocessor 910, a memory 920, a radio frequency (RF) module 930, adisplay module 940, and a user interface module 950.

The communication apparatus 900 is illustrated for convenience ofdescription, and some of its modules may be omitted. Also, thecommunication apparatus 900 may further include necessary modules.Moreover, some modules of the communication apparatus 900 may be dividedinto segmented modules. The processor 910 is configured to perform theoperation according to the embodiment of the present inventionillustrated with reference to the drawings. In more detail, the detailedoperation of the processor 910 will be understood with reference to thedescription of FIG. 1 to FIG. 8.

The memory 920 is connected with the processor 910 and stores anoperating system, an application, a program code, and data therein. TheRF module 930 is connected with the processor 910 and converts abaseband signal to a radio signal or vice versa. To this end, the RFmodule 930 performs analog conversion, amplification, filtering andfrequency uplink conversion, or their reverse processes. The displaymodule 940 is connected with the processor 910 and displays variouskinds of information. Examples of the display module 940 include, butnot limited to, well-known elements such as a liquid crystal display(LCD), a light emitting diode (LED), and an organic light emitting diode(OLED). The user interface module 950 is connected with the processor910, and may be configured by combination of well known user interfacessuch as keypad and touch screen.

The aforementioned embodiments are achieved by combination of structuralelements and features of the present invention in a predetermined type.Each of the structural elements or features should be consideredselectively unless specified separately. Each of the structural elementsor features may be carried out without being combined with otherstructural elements or features. Also, some structural elements and/orfeatures may be combined with one another to constitute the embodimentsof the present invention. The order of operations described in theembodiments of the present invention may be changed. Some structuralelements or features of one embodiment may be included in anotherembodiment, or may be replaced with corresponding structural elements orfeatures of another embodiment. Moreover, it will be apparent that someclaims referring to specific claims may be combined with another claimsreferring to the other claims other than the specific claims toconstitute the embodiment or add new claims by means of amendment afterthe application is filed.

The embodiments according to the present invention may be implemented byvarious means, for example, hardware, firmware, software, or theircombination. If the embodiment according to the present invention isimplemented by hardware, the embodiment of the present invention can beimplemented by one or more application specific integrated circuits(ASICs), digital signal processors (DSPs), digital signal processingdevices (DSPDs), programmable logic devices (PLDs), field programmablegate arrays (FPGAs), processors, controllers, microcontrollers,microprocessors, etc.

If the embodiments according to the present invention are implemented byfirmware or software, the embodiments of the present invention may beimplemented by a type of a module, a procedure, or a function, whichperforms functions or operations described as above. A software code maybe stored in a memory unit and then may be driven by a processor. Thememory unit may be located inside or outside the processor to transmitand receive data to and from the processor through various means whichare well known.

It will be apparent to those skilled in the art that the presentinvention can be embodied in other specific forms without departing fromthe spirit and essential characteristics of the invention. Thus, theabove embodiments are to be considered in all respects as illustrativeand not restrictive. The scope of the invention should be determined byreasonable interpretation of the appended claims and all change whichcomes within the equivalent scope of the invention are included in thescope of the invention.

Although the aforementioned method for transmitting and receivingdownlink control information in a wireless communication system and theapparatus for the same have been described based on the 3GPP LTE system,they may be applied to various wireless communication systems inaddition to the 3GPP LTE system.

What is claimed is:
 1. A method for receiving downlink channels from abase station (BS) at a user equipment (UE) in a wireless communicationsystem, the method comprising: detecting control information bymonitoring a physical downlink control channel (PDCCH) or an enhancedphysical downlink control channel (EPDCCH); and receiving a broadcastchannel or a downlink shared channel based on the control information,wherein the PDCCH is monitored on subframes configured for the broadcastchannel, and wherein the EPDCCH is not monitored on the subframesconfigured for the broadcast channel.
 2. The method of claim 1, whereinthe PDCCH includes the control information to receive the broadcastchannel
 3. The method of claim 1, wherein the EPDCCH includes thecontrol information to receive the downlink shared channel.
 4. Themethod of claim 1, wherein the PDCCH is monitored in a common searchspace and the EPDCCH is monitored in a UE specific search space.
 5. Auser equipment (UE) in a wireless communication system, the UEcomprising: a wireless communication module configured to receivedownlink channels from a base station (BS); and a processor configuredto: detect control information by monitoring a physical downlink controlchannel (PDCCH) or an enhanced physical downlink control channel(EPDCCH), and control the wireless communication module to receive abroadcast channel or a downlink shared channel based on the controlinformation, wherein the PDCCH is monitored in subframes configured forthe broadcast channel, wherein the EPDCCH is not monitored in thesubframes configured for the broadcast channel.
 6. The UE of claim 5,wherein the PDCCH includes the control information to receive thebroadcast channel
 7. The UE of claim 5, wherein the EPDCCH includes thecontrol information to receive the downlink shared channel.
 8. The UE ofclaim 5, wherein the PDCCH is monitored in a common search space and theEPDCCH is monitored in a UE specific search space.
 9. A method fortransmitting downlink channels to a user equipment (UE) at a basestation (BS) in a wireless communication system, the method comprising:transmitting control information via a physical downlink control channel(PDCCH) or via an enhanced physical downlink control channel (EPDCCH);and transmitting a broadcast channel or a downlink shared channel basedon the control information, wherein the PDCCH is transmitted onsubframes configured for the broadcast channel, and wherein the EPDCCHis not transmitted on the subframes configured for the broadcastchannel.
 10. The method of claim 9, wherein the PDCCH includes thecontrol information to be received the broadcast channel by the UE. 11.The method of claim 9, wherein the EPDCCH includes the controlinformation to be received the downlink shared channel by the UE. 12.The method of claim 9, wherein the PDCCH is transmitted in a commonsearch space and the EPDCCH is transmitted in a UE specific searchspace.
 13. A base station (BS) in a wireless communication system, theBS comprising: a wireless communication module configured to transmitdownlink channels to a user equipment (UE); and a processor configuredto: control the wireless communication module to transmit controlinformation via a physical downlink control channel (PDCCH) or via anenhanced physical downlink control channel (EPDCCH) to a user equipment(UE), and transmit a broadcast channel or a downlink shared channelbased on the control information to the UE, wherein the PDCCH istransmitted on subtropics configured for the broadcast channel, andwherein the EPDCCH is not transmitted on the subframes configured forthe broadcast channel.
 14. The BS of claim 13, wherein the PDCCHincludes the control information to be received the broadcast channel bythe UE.
 15. The BS of claim 13, wherein the EPDCCH includes the controlinformation to be received the downlink shared channel by the UE. 16.The BS of claim 13, wherein the PDCCH is transmitted in a common searchspace and the EPDCCH is transmitted in a UE specific search space.